Sources said the teen could be heard on the 911 call loudly grunting as he allegedly pummeled his mother.

One neighbor said other residents told him Wachtel was crying hysterically and saying, “I’m sorry, Mommy!” after the beating.

Another neighbor said the pair had a volatile relationship.

“She said terrible things to him and you would hear things fly. I’d hear her yelling, ‘I hate you’ and ‘Why are you doing this to me?’ ” said that neighbor, Leigh Miller.

“It went on for years, since Henry was a little boy.”

Midtown North cops, who rushed to the scene, found Kay covered in blood lying face-up on her kitchen floor with a fractured skull, broken eye socket and ribs.

Wachtel — who was charged with murder last night — was also covered in blood, the sources said.

He was later taken to Bellevue Hospital for a psych exam.

Kay, who wrote the screenplay for the 1988 movie “Call Me,” starring Steve Buscemi, was a visiting instructor at Pratt Institute in Brooklyn and a teacher at La Guardia HS of Music & Art and Performing Arts in Manhattan, according to Pratt’s Web site.